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Interpret SimEvents Models Using Statistical Analysis

Choosing the right statistical measure is critical for evaluating the model performance. You can use output statistics from the SimEvents^® library blocks for data analysis and run-time control.

Output Statistics for Data Analysis

Consider these statistical measures for more efficient behavior interpretation.

Identify the appropriate size of the samples to compute more meaningful statistics.
Decide if you want to investigate the transient behavior, the steady-state behavior, or both.
Specify the number of simulations that ensures sufficient confidence in the results.

For an example, see Explore Statistics and Visualize Simulation Results.

Output Statistics for Run-Time Control

Some systems rely on statistics to influence the dynamics. In this example, a queuing system with discouraged arrivals has a feedback loop that adjusts the arrival rate throughout the simulation based on the statistics reported by the queue and the server. To learn more details about this example, see Adjust Entity Generation Times Through Feedback.

Snapshot of a Simulink model that shows an Entity Generator block connected to an Entity Queue that, in turn, connects to an Entity Server block. Output signals from the Entity Server and Entity Queue blocks enter a Simulink Function block that, in turn, feeds into the Entity Generator block.

A subset of the blocks in SimEvents library provides statistics output for run-time control. When you create simulations that use statistical signals to control the dynamics, you access the current statistical values at key times throughout the simulation, not just at the end of the simulation.

This table lists SimEvents blocks that output commonly used statistics for data analysis and run-time control.

Block Name	Statistics Parameter
Block Name	Average queue length/store size, l	Average wait, w	Number of entities arrived, a	Number of entities departed, d	Number of entities extracted, ex	Number of entities in block, n	Number of pending entities, np	Pending entity present in block, pe	Utilization, util
Conveyor System
Entity Batch Creator
Entity Batch Splitter
Entity Find
Entity Generator
Entity Queue
Entity Selector
Entity Server
Entity Store
Entity Terminator
Multicast Receive Queue
Resource Acquirer
Resource Pool

The statistical parameters are updated on particular events during the simulation. For example, when a full N-server advances one entity to the next block, the statistical signal representing the number of entities in the block assumes the value N-1. However, if the departure causes another entity to arrive at the block at the same time instant, then the statistical signal assumes the value N. The value of N-1, which does not persist for a positive duration, is a zero-duration value.. This phenomenon occurs in many situations.

This table lists the events that update the block statistics.

Statistics Port	Updated on Event
Statistics Port	Entry	Exit	Blocked	Preempted	Extracted
Average queue length/store size, l
Average wait, w
Number of entities arrived, a
Number of entities departed, d
Number of entities extracted, ex
Number of entities in block, n
Number of pending entities, np
Pending entity present in block, pe
Utilization, util

Average Queue Length and Average Store Size

The formula to compute average queue length or store size

Average queue length, l is the accumulated time-weighted average queue. To compute Average queue length, l at time t_f, the block:

Multiplies the size of the queue n by its duration, t = t_i - t_i-1, to calculate the time-weighted queue.
Sums over the time-weighted queue and averages it over total time t_f.

$l = \frac{1}{t_{f}} \sum_{i = 1}^{f} n_{t} \times t$

Where:

t is the time between the entity arrival and / or the number of departure events.
f is the total number of entity arrival and / or the number of departure events between t₀ and t_f.
i = 1 for simulation time t₀ = 0.

Average store size, l is computed similarly by replacing the queue length with the store size.

Average queue length example in the Entity Queue block

Open Model

This example shows the average queue length of the entities in the Entity Queue block.

Calculate average queue length in the simple queuing system example

The service time for the Entity Server block is larger than the entity intergeneration time of the Entity Generator block. The entities are queued and sorted in the Entity Queue block. The scope displays the number of entities.

Scope block output representing number of entities, graphically.

For the duration between 0 and 1, the average queue length is 0 because the size of the queue is 0. Between 1 and 2 the queue length is 1. Average queue length at time t_f = 2 is calculated as follows.

$l = \frac{1}{2} \sum_{i = 1}^{2} n_{t} \times t = \frac{1}{2} (0 + 1 \times 1) = 0.5$

The queue size is 2 between the times 2 and 6 for the duration of 4. Average queue length at time t_f = 6 is calculated using this equation.

$l = \frac{1}{6} \sum_{i = 1}^{6} n_{t} \times t = \frac{1}{6} (0 + 1 \times 1 + 2 \times 4) = 1.5$

The average queue size is calculated for each duration. The Scope block displays its value for the duration of the simulation.

Scope block output representing average queue size, graphically.

Average Wait

The formula to compute average wait

The Average wait, w parameter represents the sum of the wait times for entities departing the block, divided by their total number, n.

Wait time, w_j, is the simulated time that an entity resides within a block. This wait time is not necessarily equivalent to the time an entity is blocked. It is the duration between an entity's entry into and exit from a block. For instance, wait time is 1 for an entity that travels through an unblocked Entity Server with a service time of 1s.

$w = \frac{\sum_{j = 1}^{n} w_{j}}{n}$

Average wait of entities example in the Entity Server block

Open Model

This example shows the average wait time for entities that are served in the Entity Server block.

Calculate average wait in the example

The duration of an entity's entry into and exit from the Entity Server block is computed by the gettime() function in the Simulink Function block.

The Diagnostic Viewer displays the duration between the entry and exit of six consecutive entities.

Snapshot of the Diagnostic Viewer dialog box that displays the time elapsed between the entry and exit of six entities, in the form of a list.

The Scope block shows the average wait time for each entity departure event from the Entity Server block. For instance, the wait time for the first entity is 1 and the wait time for the second entity is 2. The average wait time calculated for the first two entities is 1.5. The plot displays this value at the simulation time 6. For the first four entities, the sum of the wait times is 10 and the average wait time at simulation time 12 becomes 2.5.

Scope block output representing the average wait time for each entity departure from the Entity Server block, graphically.

Number of Entities Arrived

The Number of entities arrived, a parameter outputs the cumulative count for the number of entities that arrive at the block.

Number of Entities Departed

The Number of entities departed, d parameter outputs the cumulative count for the number of entities that depart the block.

Number of Entities Extracted

Entity Find block finds entities in a SimEvents model and extracts them from their location to reroute. The Number of entities extracted, ex parameter outputs the number of entities that are extracted from a block.

Number of Entities in Block

The Number of entities in block, n parameter outputs the number of entities that are in the block.

Number of Pending Entities

The Number of pending entities, np parameter outputs the number of pending entities the block has served that have yet to depart.

Pending Entity Present in Block

The Pending entity present in block, pe parameter indicates whether an entity that is yet to depart is present in the block. The value is 1 if there are any pending entities, and 0 otherwise.

Utilization

The Utilization, util parameter indicates the average time a block is occupied. The block calculates utilization for each entity departure event, which is the ratio of the total wait time for entities to the server capacity, C, multiplied by the total simulation time, t_f. Utilization for n entities is calculated using this equation.

$u t i l = \frac{\sum_{j = 1}^{n} w_{j}}{C \times t_{f}}$

References

[1] Cassandras, Christos G. Discrete Event Systems: Modeling and Performance Analysis. Homewood, Illinois: Irwin and Aksen Associates, 1993.